The ring laser gyroscope is a navigation instrument which utilizes the Sagnac effect to detect rotations. In a ring laser gyro, resonant properties of a closed cavity convert the Sagnac-induces phase difference between the clock-wise (CW) and counter clock-wise (CCW) propagating beams into a frequency difference, which is more easily measured than the absolute phase shift. This is called resonant frequency splitting. Ring laser gyroscopes may be classified as passive or active, depending upon whether the gain medium is external or internal to the cavity. In the active ring laser gyroscope the cavity defined by the closed optical path becomes an oscillator. Output beams from the two directions interfere to give a beat frequency, which is proportional to the frequency difference between the CCW and CW beams and is a proportional to the rotation rate. The constant of proportionality between the resonant frequency splitting and the rotation rate is the scale factor, which depends on the area enclosed by the ring laser. The oscillator approach has the advantage that the frequency filtering properties of the cavity resonator are narrowed by many orders of magnitude below the passive cavity to give the potential for simple configurations for very precise rotation sensing. To date, the major ring laser gyroscope rotation sensor effort has been put into the active ring laser. Presently most commercially available optical rotation sensors are active ring laser gyroscopes. However, there are undesirable effects operating in any such device such as null shift, lock-in, and mode pulling.
When the rotation rate of the ring laser gyroscope is below a certain absolute threshold, the frequency difference between the CW and CCW beams can no longer be differentiated. This phenomenon is called frequency lock-in, or mode locking. This affects the ring laser gyroscope because at low rotation rates the CW and CCW beams' frequencies can not be differentiated and the ring laser gyroscope produces a false indication that the device is not rotating. If the rotation rate of a ring laser gyroscope starts at a value above that where lock-in occurs and is then decreased, the frequency difference between the beams disappears at a certain rotation rate. This rotation rate is called the lock-in threshold. The range of rotation rates over which lock-in occurs is generally called the dead band of the ring laser gyroscope. Lock-in arises from coupling of light between the clockwise and counter clockwise beams. The coupling results primarily from backscatter off the mirrors that recirculate the beams around the closed path. Backscatter causes the beam in each direction to include a small component having the frequency of the counter propagating beam in the other direction. The lock-in effect in a ring laser gyroscope is similar to the coupling that has long been observed and understood in conventional electronic oscillators. Even with optimum design and high reflectivity mirrors the lock-in phenomenon is invariably present.
Any inability to accurately measure low rotation rates reduces the effectiveness of a ring laser gyroscope in navigational systems. Therefore it is well known that a ring laser gyroscope requires means for circumventing mode locking.
The actual active gain section contains the lasing medium that is the source of the laser radiation. Any such medium is dispersive, that is, its refractive index varies with frequency. Dispersion is classified as normal in frequency regions where the refractive index increases slowly and smoothly with increasing frequency, thus having a positive slope. Typically in a ring laser gyro the refractive index is considered constant. However, in regions near an atomic resonance (gain maximum) the index undergoes a rapid change and there is dispersion, typically accompanied by absorption. When the slope of the dispersion is negative it is called anomalous dispersion.
The source of error in laser gyroscopes is principally because of the effect on gyroscope scale factor. In a ring laser employing a gaseous gain section, anomalous dispersion in the neighborhood of resonances (where the laser operates) causes oscillating modes to be displaced in frequency from their ideal, empty cavity values. The amount of displacement varies with gain, and this mode pulling gives rise to a change in the gyroscope scale factor, that is, in the magnitude of the beat frequency due to rotation. This is a source of error.
Although lock-in cannot be completely eliminated, the problem can be made tractable through deliberate imposition of a known null shift bias. This bias provides a beat frequency in the absence of rotation that is known and that can be subtracted from the rotation readout to obtain the true rotation rate. Null shift biases are generated by several means. Included are discharge gas flow, Langmuir flow, Faraday effect, and mechanical motion of the gyro (dithering).
Dithering involves mechanically oscillating the ring laser gyroscope about its sensor axis so that the device is constantly sweeping through the deadband and is never locked therein. This mechanical oscillation of the ring laser gyroscope is produced by a dither motor attached to the ring laser gyroscope. Dither suspension and drive mechanisms are mechanically complex, and fail to completely eliminate the effects of residual backscatter coupling. Some guidance applications cannot tolerate the amount of mechanical vibration required to mechanically dither the ring laser gyroscope frame.
As shown in FIG. 1, as is known in the art a ring laser gyroscope 10 is typically formed of a block 12 provided with interior passages 14 that communicates with openings at each of its corners. Mirrors 16, 18, and 20 are provided at the corners with one of the mirrors 16, 18, and 20 being used as a read-out device. The interior passages 14 and the mirrors 16, 18, and 20 define a plasma chamber in the form of a closed laser resonant path.
A cathode 22 and anodes 24 and 26 engage corresponding surfaces of the block 12 at openings there through. Indium is usually used to form seals between the block 12 and the electrodes comprising the cathode 22 and the anodes 24 and 26. These seals confine the gas that is energized to provide the laser plasma within the plasma chamber. The energized gas is often referred to as the discharge gas. The Indium seals are compressible so that a tight seal is formed. Also, a dither motor 28 to be discussed below is provided between the block 12 and a support structure.
A source 30 supplies an electric potential across the cathode 22 and the anodes 24 and 26. Typically, the source 30 biases the anodes 24 and 26 at or slightly negative with respect to the potential of the block 12, particularly the potential in the area of the block 12 at the dither motor 28. This potential is a reference potential such as ground. The source 30 biases the cathode 22 at a potential that is more negative than the potential of the anodes 24 and 26.
Accordingly, the invention operates as follows. The electric potential across the cathode 22 and the anode 24 energizes the gas in the interior gain bore passages 14 so as to form a laser plasma that supports a laser which traverses the optical closed loop provided by the interior passages 14 in one direction such as a clockwise direction. Similarly, the electric potential across the cathode 22 and the anode 26 energizes the gas in the interior gain bore passages 14 so as to form a laser plasma that supports a laser which traverses the optical closed loop provided by the interior passages 14 in the opposite direction such as a counterclockwise direction.
One of the problems associated with the ring laser gyroscope 10 is lock-in which occurs at low rotation rates. Backscatter from the mirrors 16, 18, and 20 within the optical path formed by the interior passages 14 couples energy from one of the lasing directions into the counter-propagating laser. While this coupling is always present, at low rotation rates the oscillating frequencies of the two counter-propagating lasers lock together in a single frequency. Thus, a ring laser gyroscope can be insensitive to rotations having low rates. Accordingly, in this embodiment shown, and as described in U.S. Pat. No. 7,058,111, the dither motor 28 is provided in order to dither the ring laser gyroscope 10 because dithering mitigates lock-in.
Recently, it has been theoretically shown that an anomalous dispersion in a resonator based Sagnac gyroscope can enhance the rotation sensitivity by orders of magnitude. In a theoretical discussion and a subsequent experimental system Shahriar et al. (2007) described how the enhancement occurs if the dispersion is anomalous, characteristic of superluminal light propagation. Under their idealized model a group index varied linearly over all frequencies, and the enhancement factor is given by the inverse of the group index, was maximal when the group index was null, corresponding to the so-called critically anomalous dispersion condition (i.e. where the group velocity become infinite). For a realistic medium, the anomalous dispersion does have a limited bandwidth, but even taking this into account, Shahriar et al suggested that the intracavity medium could be manipulated such that a very large enhancement could be achieved, for example, as high as 106.
Accordingly the experimental system of Shahriar et al. described potential ultrahigh enhancement in a gyro system using Rubidium (85Rb) to produce bi-frequency gain splitting by optically pumping an intercavity vapor to produce a population inversion between the F=2 and 3 hyperfine states. Bi-frequency gain is a method for producing anomalous dispersion absent absorption of the optical beams. In general, they described how a propagation medium, in their experimental system 85Rb, was used to enhance the sensitivity of measuring absolute rotations.
In the experiments above, two pump beams of slightly different frequencies were used to generate closely spaced gain peaks, called a gain doublet, in order to produce anomalous dispersion. While this is not the only means for creating anomalous dispersion, this scheme is preferable to other means for generating anomalous dispersion because there is little or no absorption of the transient light. One beam is frequency-locked to the 5S1/2, F=2 to 5P3/2, F′=3 transition in 85Rb and orthogonally polarized with respect to the probe beam. The two pump beams and the probe are combined using two beam splitters, one polarizing, and the other non-polarizing at the input end, and made to propagate collinearly in Rb vapor cell that contains a mixture of the 85 and 87 isotopes.
A practical implementation of a RLG is to use a HeNe gas mixture as the lasing medium. Historically a mixture of Ne-20 and Ne-22 is used to eliminate mode competition between the CCW and CW beams, as is demonstrated, for example in U.S. Pat. No. 7,058,111. Importantly, although Shahriar et al. demonstrated that 85Rb works well in proof-of-concept experiment, these results are not translatable for use in a real gyro.